Agricultural tractor hydraulic systems which operate high capacity equipment typically generate a considerable amount of heat which must be dissipated. For example, an implement such as a large seeding tool with a hydraulically driven fan often includes an active hydraulic down force system which operates simultaneously with the fan. Many tractors include an additional output port, often referred to as a power beyond pressure port, which is connected to the hydraulically powered components on the implement, to provide additional hydraulic capacity.
During activation of the down force system, the hydraulic circuit must run in a high pressure standby condition. The selective control valve on the tractor is moved to an active position, such as the lower position, and as the down pressure control valve adjusts pressure to the implement cylinders the hydraulic pump receives a signal from the system indicating a stalled condition. The stall signal causes the pump to run at the high pressure condition which requires more power and generates more heat. When a large flow of oil is required by the implement, such as required by the fan, during the high pressure standby condition a large amount of hydraulic energy must be dissipated by valves in the system. This energy dissipation generates a large amount of heat energy. Under certain extreme temperature conditions, tractors with marginal hydraulic cooling systems may overheat.
A partial solution is provided in U.S. Patent Publication US2010/0078185, commonly assigned with the present application and hereby incorporated by reference. There, additional valve components are configured into the active down force circuit to cause the tractor hydraulic system to operate below the stall or high pressure standby condition. A check valve connects the tractor power beyond supply line to the pressure reduction valve that is connected to the implement cylinder ends and controls down pressure. The tractor selective control valve (SCV) is then operated at load pressure in the float mode when the down force circuit is controlling implement down pressure. The circuit eliminates a stall signal to the hydraulic pump that otherwise would cause the pump to rise to the high, heat-producing stall pressure when operating in the active pressure mode. During implement lift, a check valve allows hydraulic flow from the cylinders to bypass the pressure reduction valve. The system therefore operates at lower pressure and lower power to produce less heat and increase fuel economy.
A load sensing system keeps the system pressure at the lowest possible level. The power beyond system provides an external load sense option. However, the power beyond system does not allow the operator to control its output. There is a need to supply an externally load-sensed pressure to the system while allowing the system to be controlled by a non-externally load-sensed selective control valve. To prevent load-sensed pressure from commanding pump flow during tractor engine startup, thus creating tractor starting issues under certain circumstances, it is necessary to prevent load sense signal pressure from being communicated to the tractor during engine startup. This could be done with an electric solenoid valve but an electric signal would have to be present and made available to the circuit. A method to accomplish activation and deactivation of the down force is needed that does not require an electrical signal. Since not all tractors are equipped with power beyond, it is also necessary to be able to operate the rockshaft cylinders and a down force circuit normally, without attaching to power beyond supply, power beyond return, or load sense lines to the circuit.